1. Field of the Invention
This invention relates to compositions that can be applied onto granules of inorganic salts, particularly granules of sodium chloride, to reduce the tendency to cake on storage and to minimize fugitive dust.
2. Related Prior Art
Many hard crystalline materials, particularly inorganic salts and mineral ores, tend to contain fines or to be friable and form fines, and these fines can be a nuisance. Also, these materials tend to harden and form large, agglomerated masses upon exposure to moisture in humid environments, particularly during long periods of storage. These hardened masses are generally referred to as cakes. Some cake formations become very rigid and resistant to separation, making the material very difficult to transport and to break apart so as to facilitate its use in particular applications. In certain situations where an enormous amount of a material has formed as a solid cake, explosives may be necessary to separate the cake into particulate matter.
It is desirable to provide an anticaking agent, in either solid or liquid form, that will be effective when applied to salts in a small amount to minimize dusting and to reduce the caking tendencies for extended periods of time so that the coarse granular crystalline materials will remain easily movable, despite long term storage, and ready for use by the consumer.
Economization and automation of industrial processes for inorganic salts require the salt to be in a granular form so that it can flow freely during subsequent processing steps. In emptying storage silos containing tons of inorganic salt, for example, there is increased operational expense and potential danger on account of the tendency of the salt to cake. The salt cakes must be disintegrated by a mechanical auxiliary apparatus in order to be emptied or loaded from the silos. Additionally, during commercial use of the salt, any step of dosing or mixing agglomerated or caked salt together with other substances is difficult due to the extent of the caking.
Many attempts have been made to find ways and means to reduce or entirely eliminate the tendency of inorganic salts to harden. One of the best known processes consists of dry mixing with such salts finely divided fillers, such as silica, alkali, alkaline earth and aluminum silicates, aluminum oxide, magnesium oxide, calcium oxide or alkaline earth carbonate. These additives form a loose jacket around the individual salt crystals or granulates that in the recrystallization of the salt prevent a formation of bridges between the individual particles and therefore the potential for caking. The disadvantage of these additives is that they must be used in relatively large amounts in order to produce a suitable anticaking effect. Through the use of large amounts of these additives the products tend to lose most of their clear solubility in water.
Organic additives such as oils, glycerine, paraffins, paraffin oils, alkyl sulfonates or fatty amines, are also effective in the prevention of anticaking in organic salts. However, these additives for the most part are disadvantageous in that they can only be placed on the salt at great industrial expense. For example, a dosing of a hot salt, as it comes out of production, by means of cocofatty amines is practically impossible because of the odor troubles and danger to health associated therewith. The amines cannot be added during the salt production step, but rather must be added in an additional step after the salt cools. In the interim storage of the salt there is no protection against the possibility of caking.
It has long been known that the hardening of alkali chlorides can be reduced by an addition of complex ferrocyanides. These products are expensive and are typically utilized in large amounts based upon the total amount of alkali chloride product being treated. Additionally, the use of ferrocyanides is of some concern due to its potential toxic effect to the environment, e.g. in road salt applications where run-off of treated road salts may wind up in lakes and streams.
It is the object of the present invention to provide an effective anticaking and antidusting agent for inorganic salts that is environmentally safe and contains only metals such as alkali metal or alkaline earth metals.
It is a further object of the present invention to provide an anticaking and antidusting agent that is highly effective when added in relatively low dosage amounts to inorganic salts in either a powder form or a liquid spray form.
It is yet another object of the invention to provide a modified inorganic salt product containing the anticaking and antidusting agent that is resistant to caking when exposed to high humidity and other moisture conditions.
The aforementioned objects are accomplished by providing an agglomeration preventing or anticaking agent for inorganic salts, as well as a process for the production of such agent, wherein the agent comprises a protein, a saccharide, an alkali metal or alkaline earth metal salt and an acid. The anticaking agent of the invention is especially suited for the prevention of caking together and for preserving the flowability of salts, specifically alkali chlorides, e.g. sodium chloride or potassium chloride, and alkali chloride containing salt mixtures. The protein in the agent is preferably a prolamine, most preferably zein. The saccharide is preferably a polysaccharide, most preferably a guar gum. The salt is preferably sodium chloride, most preferably sodium chloride with trace amounts of magnesium chloride and calcium chloride. The acid in the agent is preferably an organic acid, and most preferably at least one of citric acid and ascorbic acid. In a most preferred embodiment of the invention, the anticaking agent composition comprises predominantly guar gum with small amounts of the prolamine, salt and acid.
The present invention also relates to a method of making the anticaking agent, comprising mixing a protein, a saccharide and a salt in an aqueous organic solvent that contains an acid until a complex is formed. The resultant complex is then separated from the solvent and dried so as to form the anticaking agent of the invention.
The present invention further relates to methods for applying an effective amount of the anticaking agent to an inorganic salt to prevent the modified salt mixture from caking, wherein the application of the anticaking agent to the inorganic salt can be in a solid or liquid form. A modified salt product that contains an effective mixture of the anticaking agent is also encompassed by the invention.
In accordance with the present invention, an agglomeration preventing or anticaking agent comprises a complex of a protein, a saccharide, preferably a polysaccharide, an alkali metal or alkaline earth metal salt and an acid, preferably an organic acid. The components of the complex are preferably not toxic and are environmentally friendly, unlike some of the other commercially available anticaking agents (e.g. ferrocyanide compositions).
The anticaking agents are also useful in the prevention of dust formation in normally dusty solid materials, namely particulate organic and inorganic material such as wheat, coal fines and the like. It is noted that the agent compositions disclosed in the invention for use as anticaking agents may be applied in the same manner and proportions to various materials to prevent the formation of dust at the surface of those materials.
The anticaking agent compositions of this invention may be employed by spraying the agent in liquid form onto coarse granules of salts, particularly sodium chloride or dietetic salt. The typical coarse granules are usually in the range of 1 to 5 millimeters in diameter, usually from 2 to 3 millimeters. The anticaking agents may also be employed by dry blending the agent in powder form into the salt compositions or dissolution into a salt brine prior followed by crystallization of the modified salt from solution.
Protein and polysaccharide components utilized in the present invention include a water-soluble monosaccharide or polysaccharide and a substantially water-insoluble protein. Those two components form a complex together when mixed either in granular form or in solution. Mixing those two components in an organic solvent together with the salt and acid utilized in the invention forms a unique complex in solution that is useful as an anticaking agent.
Proteins useful in formulating the anti-caking agents include any protein that is predominately or substantially water-insoluble. Vegetable proteins or prolamines are particularly preferable due to their availability and low cost. Prolamines are cereal-derived proteins that are insoluble in water, absolute alcohol or neutral solvents and soluble in dilute (80%) alcohol. Examples of suitable prolamines for use in the present invention include, but are not limited to, corn-derived prolamine or zein, barley-derived prolamine or hordein, wheat-derived prolamine or gliadin, millet and combinatons thereof. In a preferred embodiment of the invention, the vegetable protein or prolamine used in the composition is zein or corn gluten. Corn gluten is obtained from corn or maize. The zein may be physically extracted from the corn gluten or, alternatively, the entire corn gluten may be provided as a mashed paste or powder.
Examples of monosaccharides and polysaccharides that can be used to prepare the anticaking agents include, but are not limited to fructose, glucose, lactose, maltose, sucrose, water-soluble cellulose derivatives, seaweed polysaccharides such as alginate and carrageenin, seed mucilaginous polysaccharides, complex plant exudate polysaccharides such as gum Arabic, tragacanth, guar gum, pectin, ghatti and the like, and microbially synthesized polysaccharides such as xanthan gum. In a preferred embodiment, the polysaccharides are guar gum, pectin, gum Arabic and combinatons thereof. The most preferred polysaccharide for use in the present composition is guar gum, which has been found to be effective in binding and stabilizing the components of the agent.
The salts utilized in making the anticaking compositions of the invention include alkali and alkaline earth metal salts. Such salts may include chlorides, carbonates, sulphates, silicates and combinatons thereof. Particularly useful salts are sodium, magnesium and/or calcium salts, preferably sodium chloride, magnesium chloride and calcium chloride. In a most preferred embodiment, calcium chloride and magnesium chloride are provided in trace amounts within sodium chloride, wherein the weight ratio between the two is about 3:1 of calcium chloride to magnesium chloride. A typical sodium chloride salt utilized in making the anticaking agent of the invention includes about 0.06% by weight of calcium chloride and 0.02% by weight of magnesium chloride.
Any acidic compound is useful in the present invention, including inorganic acids such as carbonic acid, sulfuric acid, hydrochloric acid and the like. However, it is preferable to utilize organic acids, preferably C1 to C20 organic acids. Suitable organic acids include, but are not limited to, citric acid, malic acid, adipic acid, tannic acid, lactic acid, ascorbic acid, acetic acid, fumaric acid and the like as well as combinatons thereof In a preferred embodiment, citric acid or a combination of citric acid and ascorbic acid is utilized.
The anticaking agent is generally prepared by mixing all of the components in an aqueous organic solvent system. A complex comprising the protein, saccharide, salt and acid forms within the upon addition and adequate mixing of the components to the solvent system. Although a complex will form based upon the addition of components in any order, it is preferable to add the acid to the solvent before the protein during the preparation of the anticaking agent of the invention. The acid facilitates adequate dispersion of the protein in the solvent to ensure adequate mixing of the agent components and formation of a complex in solution. In a generally preferred method for preparing the agent, a sufficient amount of soluble monosaccharide or polysaccharide is first dissolved in an aqueous organic solvent system. A sufficient amount of acid and an alkali or alkaline earth metal salt are then added to the solution, followed by the addition of a sufficient amount of water-insoluble protein or prolamine. Upon addition of the protein and adequate mixing, a complex forms within the solution. If desired, the solvent is then separated or evaporated from the solution to yield the final anticaking agent.
The aqueous organic solvent system is a mixture containing at least one organic solvent in water. Suitable organic solvents include, but are not limited to, alcohols such as ethyl alcohol and isopropyl alcohol, glycols such as propylene glycol and polyethylene glycols, and, ketones such as acetone. In a preferred embodiment of the invention, the aqueous organic solvent system is either aqueous ethyl alcohol or aqueous isopropyl alcohol. Alcohols generally are preferred because they can hold up to six grams of the prolamine in solution for each 100 milliliters of alcohol. The desired ratio of water to organic solvent in the aqueous organic solvent system is dependent on factors such as the miscibility of the solvent in the water and the amount of protein to be dissolved. When the organic solvent system is aqueous ethyl alcohol or aqueous isopropyl alcohol, the amount of water generally ranges between about 10% to 40% by weight and the amount of alcohol generally ranges between about 60% to 90% by weight. Preferably, the amount of water in such systems is between about 25% to 35% and the amount of alcohol is between about 65% to 75%.
The preferred final composition by weight of the dried anticaking agent is as follows: between about 0.00164-99.997% of protein, preferably zein; between about 0.001-99.87% of saccharide, preferably guar gum; between about 0.001-99.5% of salt, preferably sodium chloride; and between about 0.001-10% of acid, preferably citric acid or citric acid combined with ascorbic acid. An anticaking agent will be effective in the prevention of cake formation in salts utilizing any of the previously disclosed weight ratios for each component. However, a preferred embodiment determined as being highly effective as an anticaking agent has a composition consisting of about 0.1-5% by weight of zein, about 90-96% by weight of guar gum, about 1-3% by weight of sodium chloride and about 0.5-3% by weight of citric acid or citric acid combined with ascorbic acid. Most preferably, the composition of each component by weight is as follows: about 0.5% zein, about 95.4% guar gum, about 2% sodium chloride and about 2.1% citric acid.
The monosaccharide or polysaccharide, protein or prolamine, alkali or alkaline earth metal salt and acid are added to the aqueous organic solvent system in appropriate amounts based upon the desired final composition of the agent. Since essentially the entire amount of each component added to the solvent forms a part of the complex in solvent, the amount of each component to add to the solvent can be easily determined based upon the desired final composition. A complex will thus form in the solvent based upon components being added in any amount necessary to yield a final composition falling within the previously disclosed weight percentage ranges. Typically, the total amount of components added to a solvent is less than or equal to about 500 grams per liter of solvent.
The dissolution of components in the solvent is carried out at a temperature between about 20xc2x0 C. (ambient room temperature) and about 60xc2x0 C., preferably about 30xc2x0 C., using conventional agitation methods to form the slurry solution. Mixing of the components in solution is carried out until a complete uniform mixture is attained. Typically, a mixing period of at least 10-30 minutes is required. Preferably, the components are mixed for several hours to ensure complete formation of the complex. The components in solution are mixed by conventional agitation methods including, but not limited to, manual shaking, mechanical shaking, magnetic stirring, mechanical stirring or a combination thereof. Such mixing results in the formation of the anticaking complex within the solvent.
Once the anticaking complex in solution has been prepared, the solvent may be separated or evaporated so as to yield a dry composition. Any number of conventional solvent removal techniques may be used including, but not limited to, vacuum drying, centrifugation, evaporation, freeze drying, air drying, convection oven drying or a combination thereof. The complex is typically dried to a moisture content of no more than about 0.1%. The anticaking composition can be further processed by grinding or milling to a desired mesh particle size for use as a powder and the like.
The application of the anticaking composition of the invention to an inorganic salt can be accomplished in a variety of ways including, but not limited to, dry mixing the composition with the salt, admixing the composition with a solution such as water and spraying onto the salt, and dissolution of the composition in a brine solution followed by recrystallization of the salt within the brine. In one embodiment, the composition of the present invention may be sprayed onto granular salts while these granules are in motion, e.g., while they are being tumbled or conveyed. The manner of spray application is itself well known and a matter of common knowledge in the field.
In prior art methods for preventing caking of inorganic salts with compositions such as ferrocyanides or aluminum silicon dioxides, typically amounts of 100 ppm or more of the composition in the salt were required to ensure adequate anticaking properties. The anticaking agent of the invention is added to a particular salt in an effective amount to prevent caking of the salt when exposed to moisture. The term xe2x80x9ceffective amountxe2x80x9d is defined as any amount of the agent that is effective in preventing caking of a salt treated with such agent. Although such effective amount can be greater than 100 ppm, it has been determined that the agent is equally as effective when utilizing amounts much less than 100 ppm. In fact, it is preferable in most applications to apply the anticaking agent to a salt in amounts of about 3-5 ppm or less, and best results are obtained when the agent is applied in that dosage range. This represents an economical treatment that is highly effective and which introduces a minimal adulteration to the treated salt product.
Upon adequate application of the anticaking composition of the invention to an inorganic salt in an effective amount, the resultant salt product is modified such that it resists caking in environments in which the salt is exposed to moisture. It is believed that the modified salt product is a result of a new crystal formation in the salt resulting from the addition of the anticaking composition of the invention.